| 摘要 |
PROBLEM TO BE SOLVED: To improve safety by enabling a key decipherer to easily confirm the rightfulness of a requester for key restoration. SOLUTION: For example, a user device U2 which requires key restoration sends restoration source information KA[B|SK]in a communication message to a key restorer device AG, which deciphers the key restoration information KA[B|SK] and decides whether or not data of the user device U2 regarding the user device U2 can be accessed according to the key restoration information B and an access rule which is made previously open on an open bulletin board Pbb. When the data can be accessed, the deciphering key SK obtained by the deciphering process is ciphered to send the ciphered deciphering key K2[SK] back to the user device U2, which deciphers the K2[SK] to obtain the deciphering key SK.
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